1. Field of the Invention
This invention relates to check valves, that is, fluid flow control devices which permit flow through a conduit in predominantly only on direction. More particularly, this invention relates to check valves suitable for use in circulatory assist devices such as cardiopulmonary bypass machines for use during cardiac surgery, and extracorporeal assist circuits for extended clinical cardiac support. Still more particularly, this invention relates to such check valves which can provide minimal hemolysis and thrombogenesis, and acceptably high fluid-dynamic efficiency and durability.
2. Prior Art Problems
Problems usually encountered in check valves, used in circulatory assist devices, include hemolysis, thrombsis and mechanical fatigue failure. Well-known experimental results show that accelerated hemolysis rates may be avoided if fluid shear rates are kept below 10.sup.4 sec.sup.-1. Further experimental results indicate that thrombogenesis may be significantly decreased if blood-contacting surfaces are smooth on a scale comparable to cellular dimensions and if fluid shear rates are everywhere kept above 10 sec.sup.-1. To exploit the consequent range of acceptable shear rates, it is necessary to avoid valve geometries which give rise to regions of separated, retrograde or persistently stagnated flow, such as occur in strut wakes and blood-wetted notches, on the one hand, or those which give rise to excessive shear rates, such as small orifices, on the other. Additionally, it is necessary that cyclic material stresses be kept low, and that rubbing and striking of surfaces be avoided, in order to provide sufficient device durability.
Fluidic valves generally have no moving solid parts and can have simple smooth geometry; such properties are potentially useful for use with blood. Among the fluidic check valves, also called fluid diodes by analogy with electronic components, perhaps the most attractive is the so-called vortex diode. This device is compact and simple to construct and can maintain an internal fluid motion at all times, even during the intervals between pulses of fluid conduction usually referred to as diastole, by analogy to the functions of the natural heart. Therefore, the vortex diode has been considered for use with pulsatile blood pumps. Its major drawback is that, for flow rates of physiological interest, the diodicity of the device, defined as the ratio of fluid flow resistances in the reverse and forward flow directions, is generally below 5, resulting in substantial reverse flows or regurgitation. While the detailed fluid flow in such devices is rather complex, a reasonably complete analysis indicates that even for an ideal vortex diode with no frictional losses, for a size appropriate for a circulatory assist device, the diodicity could scarcely exceed 10, so the performance limitation is fundamental. For more physiological performance, a diodicity of the order of 20, or more, would be preferred.